This invention relates to nozzle airfoil design, and more particularly, to an improved airfoil to airfoil rib joint for reducing thermal stresses at that junction.
Gas turbines typically include a compressor section, a combustor and a turbine section. The compressor section draws in ambient air and compresses it. Fuel is added to the compressed air in the combustor and the air fuel mixture is ignited. The resultant hot fluid enters the turbine section where energy is extracted by turbine blades, which are mounted to a rotatable shaft. The rotating shaft drives the compressor in the compressor section and drives, e.g., a generator for generating electricity or is used for other functions. The efficiency of energy transfer from the hot fluid to the turbine blades is improved by controlling the angle of the path of the gas onto the turbine blades using non-rotating, airfoil shaped vanes or nozzles. These airfoils direct the flow of hot gas or fluid from a nearly parallel flow to a generally circumferential flow onto the blades. Since the hot fluid is at very high temperature when it comes into contact with the airfoil, the airfoil is necessarily subject to high temperatures for long periods of time. Thus, in conventional gas turbines, the airfoils are generally internally cooled, for example by directing a coolant, which is compressed air in some systems and/or nozzle stages and steam in others, through internal cooling cavities in the airfoil.
Inside the airfoil, ribs are conventionally provided to extend between the convex and concave sides of the airfoil to provide mechanical support between the concave and convex sides of the airfoil. The ribs are needed to maintain the integrity the nozzle and reduce ballooning stress of the cavities. The ribs concurrently define at least part of the coolant flow path(s) through the airfoil. Thus, during engine operation, the internal ribs will be at a temperature level close to that of the coolant flowing through the airfoil, while the peripheral airfoil metal will generally be at a much higher temperature level. The mismatched temperatures result in high thermal stresses at the junctures of the ribs and the airfoil sidewalls. This high stress level combined with the high operating temperature results in fast deterioration of the vane at that area and thus deteriorated component life.
The invention is embodied in a vane or nozzle airfoil structure in which one or more of the nozzle ribs are connected to the airfoil side walls in such a way that the ribs provide the requisite mechanical support between the concave side and convex side of the airfoil but are not locked in the radial direction of the vane or nozzle assembly, longitudinally of the vane or airfoil. This configuration minimizes the stress caused by the mismatch of the material temperature between the airfoil outer, side walls and the support ribs.
The mechanical support ribs may be bi-cast onto a preformed nozzle airfoil side wall structure or fastened to the airfoil by an interlocking slide connection and/or welding. This substantially independent formation and mechanical interconnection enables some play in the radial direction of the nozzle assembly, longitudinally of the airfoil. By attaching the nozzle ribs to the nozzle airfoil metal in such a way that allows play longitudinally of the airfoil, the temperature difference induced radial thermal stresses at the nozzle airfoil/rib joint area are reduced while maintaining proper mechanical support of the nozzle side walls.